Vehicles may be fitted with fuel vapor recovery systems wherein vaporized hydrocarbons (HCs) released from a fuel tank (for example, during refueling) are captured and stored in a fuel vapor canister packed with an adsorbent, such as charcoal or carbon. At a later time, when the engine is in operation, the fuel vapor recovery system may use a vacuum (or pressure) to purge the vapors into the engine intake manifold for use as fuel. The purge flow vacuum (or pressure) may be generated by one or more pumps and/or ejectors or by pressures in the engine intake manifold.
However the inventor herein has recognized that while it may be desirable for a fuel vapor canister to have a low flow restriction for venting, e.g., during refueling events, this is counter to the other function of the canister that is to allow purge of the canister. For example, during a fuel vapor purge event, if flow restriction through the canister is low then a substantial amount of fuel vapor may remain in the canister after the purge leading to an increase in bleed emissions, for example. For example, during a fuel vapor purging event with low restriction in the canister, corners or edges of the fuel vapor canister may not be cleaned out.
In order to address these issues, in one example approach a method for an engine with a fuel vapor recovery system is provided. The method comprises increasing an amount of flow restriction between storage material in a fuel vapor canister while maintaining a vent valve open during a fuel vapor purging event.
In this way, a low flow restriction in the canister may be present during venting events whereas an increased flow restriction between storage material in the canister may be used during fuel vapor purging to sufficiently purge the fuel vapor stored in the canister. Further, by changing flow restriction between storage material in the canister, e.g., via restricting communication between charcoal beds of the canister, an increase in purging efficiency may be obtained without relying on changing air flow or vacuum provided to the canister by the engine during the purging event. Increased efficiency of fuel vapor purging may lead to lower bleed emissions, for example.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.